The present invention relates to a grate for a fuel boiler, consisting of plates arranged stepwise or in a cascading fashion, with at least two fixed plates and a third movable plate positioned therebetween. The third plate is movable to and fro and is provided with an appropriate drive. Combustion air passes through the interspace between the plates.
In combustion furnaces with continuous or intermittent fuel feed, the aim is the most complete combustion of the fuel possible, in order to minimize ash production. For this purpose, it is necessary to allow the fuel to remain on the grate for a relatively long time, in order to provide the burning time necessary for complete combustion. In order to create uniform combustion conditions, it is then necessary to transport the fuel from one end of the grate to the opposite end of the grate, so that fresh fuel can be resupplied to establish a reasonably uniform state. For the transport of the fuel from one end of the grate to the other, it is desirable that no fuel fraction can drop through the grate before reaching the ash-side end of the grate, since the fuel dropping through may not yet have been completely burned.
A typical grate of the type mentioned consists of fixed plates arranged stepwise, between each of which a plate movable to and fro is arranged. The movable plates are of hollow design and carry a water flow, which is intended to protect the plate material. Primary air for the combustion flows through the interspace between the movable and fixed plates, while secondary air can enter at the height of the fuel bed through lateral orifices in the boiler wall. Due to the movement of the movable plates of the grate, the fuel migrates down on the steps of the grate until it drops into an ash receiver at the lower end of the grate. The combustion in the firing chamber is assisted by a flame directed from a side wall of the boiler upon the grate, which flame is fed by another fuel, for example gas. In order to establish an air supply which is adapted to the fuel quality varying during the migration over the grate, various chambers are provided underneath the grate, into which combustion air can be blown at different rates, which air then exits through the respective interspace of the grate. A water-cooled heat shield arranged at an incline above the grate ensures relatively fast cooling of the combustion gases above the grate. The movable and fixed plates of the grate are arranged such that only one gap is formed between every two stationary plates, because one stationary plate and one fixed plate are in direct mutual contact in each case.
The known grate has numerous disadvantages. The use of primary and secondary air does not allow fully controlled combustion. The same applies to the water cooling of the movable grate plates, which does not permit complete combustion of a type of fuel which is difficult to burn, such as trash. Furthermore, the movable plates mounted on the fixed plates are subject to considerable wear, since solid fuel particles damage the surface thereof during the relative movement of the two superposed plates. A further disadvantage of the known grate is that the fuel drops down a slope upon the grate. It is therefore entirely a matter of chance how far the fuel cascades on the grate before it comes to a stop. It is clear that this results in widely different residence times of fuel particles on the grate.
The known boiler and grate arrangement therefore allows neither a complete nor adequately controlled combustion of the fuel. Furthermore, the known grate arrangement does not make possible a marked reduction in the pollutant content of the combustion gases.